In order to ensure high availability and operational reliability of the electrical power supply and to ensure personnel safety in the area of electrical installations, supply networks are increasingly used, the active parts of which are separated from the ground potential. In this type of supply network, designated as ungrounded IT system, an active conductor can have an insulation fault without the need to interrupt the running operation of the installation because due to the ideally infinite impedance value between conductor and ground, no closed circuit can form in this first fault case. An insulation fault is to be understood here as a faulty state of the IT system resulting in the insulation resistance falling below the permissible insulation level. It follows from this view that the resistance in a monitored network, including the resistances against ground (insulation resistance) of all equipment connected thereto, has to be constantly monitored because through a potential further fault at another active conductor (second fault), a fault loop would be generated and the residual current flowing in this case in connection with an overcurrent protection would result in a shut-off of the installation. By means of a constant insulation monitoring of the ungrounded IT system, a falling insulation resistance can be detected at an early stage and can be reported. The operator of the installation thereby gains an information advantage which gives the operator sufficient time to fix the (first) fault and therefore to achieve high availability and operational reliability of the installation.
Besides these ungrounded IT systems, IT systems which are connected to ground through a sufficiently high impedance are also permissible according to the DIN VDE standard 0100-410 (VDE 0100-410):2007-06 or the IEC mirror standard IEC 60364-4-41 in paragraph 411.6. This artificial impedance can also be implemented as ohmic resistance—hereinafter also designated as functional grounding resistance—and can be configured to be high-ohmic or low-ohmic. In the case of a high-ohmic configuration, the functional grounding resistance limits an occurring residual current to a value which does not yet trigger an overcurrent protection.
This type of resistance grounding is internationally also designated as high-resistance grounding (HRG) and has been proposed in the draft standard IEC 60364-7-712:2011 for photovoltaic installations as “functional earthing”. Circuitry-wise, due to its arrangement between active conductor and ground, such a functional grounding resistance (HRG resistance) can be represented as a resistor connected in parallel to the actual insulation resistor. Said parallel connection results in a total insulation resistance which is measurable, but does not allow to make any assessment about that portion of the insulation resistance reduction that is caused by a change of the actual insulation resistance, thus caused by a change of the resistance in the monitored network, including the resistances against ground of all equipment connected thereto. An evaluation of the actual insulation status of the installation is therefore only possible through an assessment on the total insulation resistance.
With a differential current monitoring device (RCM) or a residual current device (RCD) connected in series with the HRG resistor, only differential currents can be detected which are based on an asymmetric insulation resistance reduction; a symmetric insulation reduction at active positive and negative poles of a direct current voltage IT network cannot be detected because these appliances do not superimpose an active measuring voltage on the network.